EP2190082B1 - Surface emitting semi-conductor laser diode and method for manufacturing the same - Google Patents

Description

The present invention relates to a surface emitting semiconductor laser diode according to the preamble of claim 1 and a method for producing such a surface emitting semiconductor laser diode.

State of the art

Surface-emitting laser diodes (Vertical Cavity Surface-Emitting Lasers - VCSEL) are semiconductor lasers in which the light emission takes place perpendicular to the surface of the semiconductor chip. Compared to conventional edge-emitting laser diodes, the surface-emitting laser diodes have several advantages, such as low electrical power consumption, the ability to directly inspect the laser diode on the wafer, ease of coupling to a fiber, longitudinal single-mode spectra, and the ability to interface the surface-emitting laser diodes to a two-dimensional one Matrix.

Generic surface emitting semiconductor laser diodes with an emission wavelength λ (= vacuum wavelength) have a resonator comprising at least a first Bragg reflector layer sequence (DBR), an active zone having a pn junction, and a second Bragg reflector layer sequence. Such laser diodes As a rule, they have a cylindrically symmetrical structure and, because of their design and their production processes, have no preferred direction for the polarization direction of the radiated wave. There are therefore two orthogonal states with respect to the polarization direction of the radiated wave. In an ideal laser structure, these two states are energetically degenerate and on an equal footing for laser operation. However, due to the electro-optic effect and anisotropies in the device design as well as asymmetries and fluctuations of the manufacturing process, this degeneracy is abolished and the VCSEL oscillates dominant only on the respectively preferred polarization mode. In most cases, the mechanism leading to a preference for a particular mode is difficult to control or obscure, resulting in an overall statistical character of the polarization behavior. The polarization jumps generally limit the use in polarization-dependent optical systems. For example, such jumps in the optical data transmission lead to increased noise. Since many applications rely on polarization-stable lasers as light sources, this means a significant reduction of the production yield. Although a preferred direction may be definable in some cases, degeneracy cancellation is not strong enough to ensure polarization stability under varying environmental and operating conditions. In this case, even slight changes of these parameters can cause a change between the two states (pole flip).

In the past, various ways to stabilize polarization have been studied. To achieve of polarization stability of GaAs-based VCSEL, growth on higher indexed [311] substrates was successfully detected, cf. For this " On 850-nm InAlGaAs Strained Quantum-Well Vertical-Cavity Surface-Emitting Lasers Grown on GaAs (311) B Substrates with High-Polarization Stability ", IEEE Photon., Technol. Lett., 12, 942 (2000 ). Since, however, the other laser properties generally deteriorate and, in particular, there are difficult growth conditions for InP-based semiconductor layers, this method is not suitable for long-wavelength VCSELs.

Another approach involves applying dielectric or metallic grating structures having periods in the wavelength range to the coupling-out mirror. This is for example in the publication " Polarization stabilization of vertical-cavity top-surface-emitting lasers by inscription of fine metal-interlaced gratings ", Appl. Phys. Lett., 66, 2769 (1995 ) or the DE 103 53 951 A1 described. The dielectric grids used lead to interference effects, whereby the total reflection is polarization-dependent amplified or attenuated by the grid. The period of the corresponding grating structures must therefore be more than half a vacuum wavelength.

In addition, it is also known to apply metal / dielectric or metal / semiconductor structures with periods smaller than the wavelength of the VCSEL to one of the two Bragg reflector layer sequences in order to generate a birefringence polarization. This is for example in the publication " Polarization control of vertical-cavity surface emitting lasers using a birefringent Bragg Reflector ", IEEE J. Sel. Top. Quantum. Electron. 1, 667 (1995 ) or the JP 80 56 049 (A ) disclosed. In this solution, metal-dielectric gratings are used to generate birefringence in the laser resonator. This should match the optical resonator length or the resonant frequency of the laser resonator only at one polarization with the maximum reflectivity of the Bragg or the. The other polarization should be suppressed. However, the experimental results described show that the achievable polarization orientation is insufficient.

While the embodiments described above have a periodic structure for polarization control outside the inner resonator region (the latter corresponds to the region between the two Bragg reflector layers) JP 2005 039102 A a structure in which the periodic grating is located between the two Bragg reflector sequences. On the other hand, this structure consists of two separate and independent components, with the periodic lattice not being in direct contact with either of the two Bragg reflector lobe sequences. Problems that are typical in the production of one-piece or monolithic diodes, such as diffusion, therefore, do not occur in this embodiment.

Another embodiment of a periodic grating is shown in FIG US 2003/0048827 A1 described. Here, a monolithic structure is used in which the grid is epitaxially integrated into one of the two semiconductor Bragg reflector layer sequences. The The alternating layer system is based primarily on a different doping. However, only comparatively small refractive index differences can be produced hereby, which does not result in sufficient polarization mode splitting.

Another approach to polarization stabilization pursues the GB 2 428 887 A , In a generic semiconductor diode photonic crystals are used for polarization stabilization. However, photonic crystals differ significantly from sub-wavelength gratings, so that as a result, the achievable polarization stability is not optimal.

Starting from this prior art, the object is thus to improve the polarization orientation and polarization stability in surface-emitting semiconductor laser diodes.

To achieve this object, a surface emitting semiconductor laser diode and a method for their preparation with the features of the independent claims are proposed. Advantageous embodiments are the subject of the dependent claims and the following description.

Advantages of the invention

The present invention presents a polarization stable VCSEL. In particular, the invention includes the integration of a periodic structure with a defined orientation, geometry and a defined refractive index profile within the monolithic VCSEL resonator in the form of a subwavelength grating: SWG). The structure enables deterministic and stable polarization behavior with high polarization mode side-mode rejection and insensitivity to feedback. In addition, the laser diode according to the invention also prefers the lateral basic mode, even at relatively large apertures, which results in a higher total Achieve single-mode performance as compared to conventional VCSELs.

In the known solutions using a grating or a periodic structure for polarization, the grating structure is mounted on the outside of the Auskoppelspiegel, whereby the quality of the reflection and polarization properties to be achieved is limited.

According to the invention, it has been recognized that a periodic structure of semiconducting material on the one hand and dielectric material on the other hand is suitable for being arranged inside the resonator. The structure has a sufficiently large refractive index contrast in order to predetermine a preferred polarization direction by means of birefringence.

Advantageously, the period of the periodic structure is at most one, preferably at most half an emission wavelength. Thus, interference effects which are not desired in the present application can be suppressed. In this application, the term "emission wavelength" is always understood to mean the vacuum wavelength.

In an embodiment, the period of the structure is at most λ / n, preferably at most λ / 2n, where n is a function of the refractive index of the semiconducting material and / or the refractive index of the dielectric material. For example, n corresponds to a so-called effective refractive index, which is calculated in particular as the average of the two refractive indices, preferably arithmetically weighted. It is likewise preferred if n is the larger of the two refractive indices equivalent. In such an embodiment, the polarization-dependent averaging of the refractive index is particularly pronounced. At an emission wavelength of 1310 nm and a (larger) refractive index of, for example, n = 3.2 for InP as a semiconductor, this results in a period length of at most ~400 nm. In the case of a diffraction grating-like structure (ie a grid of regularly spaced longitudinal structures), the duty cycle (Ratio of bar and pit width) 1: 1 or have deviating values. In the above example, the land and pit widths could each be 200 nm. It is expected that the smaller the period length, the better the polarization stability.

In contrast to the lattice structures described in the prior art, significantly smaller periods in the sub-wavelength range are therefore preferably used (typically less than half the emission wavelength), since it depends on the generation of a polarization-dependent effective refractive index and not on the polarization-dependent increase of the mirror reflectivity with larger periodic gratings.

According to a preferred embodiment, the periodic structure directly adjoins the second Bragg reflector layer sequence. The present embodiment thus teaches, in particular, to produce an effective birefringence by arranging a semiconductor / dielectric structure within the resonator, wherein the structure is placed in front of a metallic, dielectric or hybrid metallic-dielectric mirror.

Preferably, the dielectric material of the periodic structure is equal to the material of the adjacent layer of the second Bragg reflector layer sequence. In this way, a simple production is possible, wherein preferably on a semiconductor layer which is to comprise the semiconductive component of the periodic structure, a mask is first defined. This is done by relevant nanostructure techniques such as electron beam lithography, nanoimprint, holography, etc. Subsequently, for example, by a dry chemical etching, the previously defined structure is transferred into the semiconductor. The etching depth is chosen so that there is a stable preferred direction of polarization. The etching depth is, for example, ~200 nm for a long-wave InP-based VCSEL. The etching depth may be in particular between 0.1 and 2, preferably between 0.5 and 1, preferably about 0.5, period lengths. After removal of the resist mask, the exposed structure is vapor-deposited with a dielectric material which immediately transitions into a Bragg-mirror layer sequence, for example of ZnS / CaF ₂ or a-Si / CaF ₂ . Unlike a conventional lattice-less VCSEL, both the thickness of the semiconductive layer and the thickness of the first dielectric mirror layer should be modified to achieve maximum polarization stability. For this purpose, the reflection within the Bragg mirror should be in phase for reflection at the interface between structure and Bragg mirror.

Conveniently, the second Bragg reflector layer sequence comprises a number of amorphous dielectric layers. As a result, the production can be simplified. To increase the reflectivity can a final layer, for example of gold, be applied. The second Bragg reflector layer sequence is usually the back mirror of the VCSEL. The coupling-out mirror of the VCSEL is usually applied directly to the substrate during production as an epitaxial layer sequence.

According to a preferred embodiment, the resonator further comprises a tunnel contact layer on the p-side of the active zone. By far the best results for long-wavelength VCSELs in the wavelength range above 1.3 μm in terms of power, operating temperature, single-mode power and modulation bandwidth have, in particular InP-based, BTJ-VCSEL (Buried Tunnel Junction, BTJ), because by using a BTJ among others the current flow can be limited to the actual area of the active zone. Conveniently, a p-confinement layer is arranged between the active zone and BTJ. It is likewise expedient to arrange an n-confinement layer on the side (n-side) of the active zone facing away from the BTJ. These confinement layers are to form barriers for the charge carriers injected during operation in order to increase the residence time of the charge carriers in the active zone.

Suitably, a dimension, for example, a length or a diameter, or the area of the projection of the periodic structure on the tunnel contact layer corresponds to at least one dimension or the area of an aperture of the tunnel contact layer. The generation of the laser radiation takes place essentially in the area defined horizontally by the aperture of the tunnel contact layer. In order to obtain a good polarization stability, should the periodic structure extending at least on this surface.

It is advantageous if the tunnel contact layer adjoins an n-doped semiconductor layer. This semiconductor layer can serve to supply power and contacting the BTJ. At the same time, this semiconductor layer may adjoin or include the semiconducting material for the periodic structure so that the semiconductor layer directly merges into the periodic structure. Reference should also be made to the above statements on a thickness adaptation of the semiconductor layer.

According to the invention, a resonator comprising at least a first Bragg reflector layer sequence, an active zone having a pn junction and a second Bragg reflector layer sequence are applied to a substrate. Furthermore, a periodic structure of semiconducting material, on the one hand, and dielectric material, on the other hand, is applied within the resonator, the main extension plane of which is arranged essentially perpendicular to the emission direction. With the method according to the invention, a laser diode according to the invention can be produced particularly reliably or reproducibly and with high quality.

In a preferred embodiment, first the n-doped epitaxial Bragg mirror formed first Bragg reflector layer sequence, then an n-doped confinement layer, then the pn junction having an active zone, then applied a p-doped confinement layer and then a tunnel contact layer. After that, the Tunnel contact layer structured to produce an aperture. Subsequently, a semiconductor layer is deposited, which is patterned to define the periodic structure. Subsequently, the second Bragg reflector layer sequence is applied. This preferred manufacturing process results in high quality laser diodes with good beam power and polarization stability.

It is understood that the features mentioned above and those yet to be explained below can be used not only in the particular combination given, but also in other combinations or in isolation, without departing from the scope of the present invention.

The invention is illustrated schematically with reference to an embodiment in the drawing and will be described below in detail with reference to the drawings.

figure description

FIG. 1

schematically shows the layer structure of a first preferred embodiment of a laser diode according to the invention.

FIG. 2

schematically shows the layer structure of a second preferred embodiment of a laser diode according to the invention.

FIG. 3

shows the time sequence of the individual steps for producing a periodic structure suitable for the invention.

FIG. 4

shows the periodic structure according to FIG. 3 schematically from above.

Below are the first FIGS. 1 and 2 cross-described characters, with the same elements are provided with the same reference numerals.

In the FIGS. 1 and 2 a first and second preferred embodiment of a laser diode according to the invention is shown schematically in a sectional view and designated 100 and 200 in total. The laser radiation occurs in both FIGS. 1 and 2 in the direction designated A from.

In the following, in particular, the structure of the resonator, which is limited by two Bragg mirrors 20 and 90, explained in more detail.

Starting from an InP substrate 10, a first Bragg reflector layer sequence formed here as an n-doped epitaxial Bragg mirror 20, an n-doped confinement layer 30, an active zone 40 and a p-doped confinement layer 50 are successively applied in a first epitaxial growth process , The Bragg mirror 20 consists of an epitaxial DBR with a reflectivity> 99%. % results. The structure is completed by the growth of a tunnel contact layer 60 consisting, for example, of a highly p ⁺ and n ⁺ -doped InGaAs layer, which is located in a node (minimum) of the longitudinal field. In the subsequent lithographic and etching process, a free-dimensional aperture D1 is generated in the layer 60, which extends either to the layer 50 or within the p-doped portion of layer 60 ends. Typical etch depths are 20 nm here.

In a second epitaxy step, an upper n-doped current supply layer 70, preferably made of InP, and an optional n-contact layer 75, preferably made of highly n-doped InGaAs, are deposited. Subsequently, a periodic structure 80 is generated. For this purpose, a mask is defined on the exposed semiconductor layer 70, for which, for example, nanostructure techniques such as electron beam lithography, nanoimprint or holography are suitable. Subsequently, for example by a dry-chemical etching, the previously defined structure is transferred into the semiconductor layer 70. The etching depth is, for example, ~200 nm for a long-wave InP-based VCSEL. After removal of the resist mask, the exposed grating is subsequently vapor-deposited with a second Bragg reflector layer sequence formed here as dielectric Bragg mirror 90, for example of ZnS / CaF ₂ or a-Si / CaF ₂ , The periodic structure 80 is thus composed of portions of the semiconductor layer 70 and portions of the first and adjacent dielectric layers 90a, respectively. The main extension plane of the structure 80 is arranged substantially perpendicular to the emission direction A of the semiconductor laser diode 100.

In the FIG. 2 illustrated, second preferred embodiment 200 differs from the laser diode 100 in that there is a reversal of the layer sequence. After application of the second Bragg reflector layer sequence 90, this is terminated by a gold layer 210, resulting in a reflectivity of almost 99.9%. Subsequently, the substrate becomes (Substrate 10 acc. FIG. 1 ), for example, by known etching techniques removed. For contacting the laser diode 200, a, for example, annular, contact layer 220 is applied. D1 denotes a dimension (aperture) of the tunnel contact layer 60, D2 denotes a dimension of the periodic structure 80.

The manufacture and the shape of a preferred embodiment of the periodic structure 80 will be described below with reference to FIGS Figures 3 and 4 explained in more detail. Here, the manufacturing process of the periodic lattice structure in FIG. 3 shown schematically from top to bottom. FIG. 4 shows a vertical view of the lattice plane.

A lacquer layer 300 is first applied to the exposed semiconductor layer 70 and the structure to be produced is transferred into it. For this purpose, known nanostructure techniques such as electron beam lithography, nanoimprint or holography are suitable. After the development of the paint, the predefined structure is obtained.

Subsequently, the predefined structure is transferred into the semiconductor layer 70 by an etching process, and the resist layer 300 is subsequently removed. In the example, a diffraction grating-type periodic structure having a period length P and a duty ratio of 1: 1 is obtained, so that the land widths L2 correspond to the pit widths L1. The etching depth H is chosen so that a stable preferred direction of the polarization results. In the present example, the etch depth corresponds to about 0.5 P.

In the example shown, the period P of the structure λ / n, where λ is the emission wavelength of the laser and n is the larger of the two refractive indices involved. With an emission wavelength λ = 1310 nm and an InP refractive index n = 3.2, this results in a period length of at most ~400 nm. Thus, the ridge widths L2, the pit widths L1 and the etching depth H are approximately 200 nm each.

After etching, the resist mask 300 is removed and covers the exposed structure with a dielectric layer 90a, which simultaneously represents the first layer of a dielectric Bragg mirror 90, for example, from ZnS / CaF _2, or a-Si / CaF. ₂ The lateral extent D2 of the grating 80 should correspond at least to the extent D1 of the buried tunnel contact 60.

It is understood that in the illustrated figures, only particularly preferred embodiments of the invention are shown. In addition, any other embodiment, in particular by a different arrangement or a different structure of the layers, etc. conceivable, without departing from the scope of this invention. The inventive structure can be applied to (BTJ) VCSEL in various material systems. These include u.a. GaAs, InP or GaSb based devices.

Claims (15)

1. Surface-emitting semiconductor laser diode comprising a resonator with a first Bragg reflector layer sequence (20), an active zone (40) comprising a pn transition, said zone (40) being embedded in a semiconductor layer sequence (30, 50, 60, 70), and with a second Bragg reflector layer sequence (90), the semiconductor laser diode (100; 200) having an emission wavelength λ,
   characterised by
   a periodic structure (80) of semiconducting material (70) and dielectric material (90a) arranged inside the resonator, embodied as a sub-wavelength grating, the main plane of extent of which is disposed substantially perpendicularly to the direction of radiation (A) of the semiconductor laser diode (100; 200) and which is in direct contact with at least one of the semiconductor layers (30, 70) embedding the active zone (40) and with at least one of the two Bragg reflector layer sequences (20, 90).
2. Semiconductor laser diode according to claim 1, wherein the periodic structure consists of at least one material of the semiconductor layers (30, 70) and at least one material of one of the two Bragg reflector layer sequences (20, 90).
3. Semiconductor laser diode according to claim 1 or 2, wherein the period (P) of the periodic structure (80) is at most λ/2.
4. Semiconductor laser diode according to claim 1, 2 or 3, wherein the period (P) of the periodic structure (80) is at most λ/n, preferably at most λ/2n, where n is a function of the refractive index of the semiconducting material (70) and/or of the refractive index of the dielectric material (90a).
5. Semiconductor laser diode according to claim 4, wherein n is the greater of the two refractive indices.
6. Semiconductor laser diode according to one of the preceding claims, wherein the periodic structure (80) is immediately adjacent to the second Bragg reflector layer sequence (90).
7. Semiconductor laser diode according to claim 6, wherein a material of the periodic structure (80), particularly the dielectric material, is identical to the material of the adjacent layer (90a) of the second Bragg reflector layer sequence (90).
8. Semiconductor laser diode according to one of the preceding claims, wherein the second Bragg reflector layer sequence (90) is an alternating layer system consisting of at least two materials with different refractive indices.
9. Semiconductor laser diode according to one of the preceding claims, wherein the resonator further comprises a tunnel contact layer (60) on the p side of the active zone (40).
10. Semiconductor laser diode according to claim 9, wherein a dimension (D2) or the area of the projection of the periodic structure (80) on the tunnel contact layer (60) corresponds at least to a dimension (D2) or the area of an aperture of the tunnel contact layer (60).
11. Semiconductor laser diode according to claim 9 or 10, wherein the tunnel contact layer (60) is adjacent to an n-doped semiconductor layer (70).
12. Semiconductor laser diode according to claim 11, wherein the periodic structure (80) is directly adjacent to the n-doped semiconductor layer (70).
13. Semiconductor laser diode according to claim 12, wherein the semiconductor material of the periodic structure (80) is identical to the material of the adjacent n-doped semiconductor layer (70).
14. Method of producing a semiconductor laser diode (100; 200) comprising the following steps:

    applying a resonator comprising at least one first Bragg reflector layer sequence (20), an active zone (40) comprising a pn transition, said zone (40) being embedded in a semiconductor layer sequence (30, 50, 60, 70), and a second Bragg reflector layer sequence (90) to a substrate (10),

    applying a sub-wavelength grating (80) as a periodic structure (80) of semiconducting (70) and dielectric material (90a), the main plane of extent of which is disposed substantially perpendicularly to the direction of radiation (A) of the semiconductor laser diode (100; 200), within the resonator in direct contact with at least one of the semiconductor layers (30, 70) embedding the active zone (40), and with at least one of the two Bragg reflector layer sequences (20, 90).
15. Method according to claim 15 comprising the following steps:

    applying the first Bragg reflector layer sequence embodied as an n-doped epitaxial Bragg mirror (20),

    subsequently applying an n-doped confinement layer (30),

    subsequently applying the active zone (40) comprising a pn transition,

    subsequently applying a p-doped confinement layer (50),

    subsequently applying a tunnel contact layer (60),

    subsequently structuring the tunnel contact layer (60) in order to produce an aperture,

    subsequently applying a semiconductor layer (70),

    subsequently structuring the semiconductor layer (70) in order to define the periodic structure (80),

    subsequently applying the second Bragg reflector layer sequence (90).

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